Oxygen (O₂) and carbon dioxide (CO₂), which are dissolved in the feedwater, are highly corrosive gases. Thus, they must be neutralized so as to prevent corrosion of the metallic components of the steam generator. By means of thermal degassing, CO₂ can be removed from the feedwater at a temperature of at least 65 °C and O₂ at 102 °C and above.

The released gases are then vented out from the top of the deaerator dome. This type of deaerator, which operates at 105°C, must be placed 5m above the ground in order to prevent cavitation in the feedwater pump. If the temperature of the condensate tank can reach 80–90 °C due to the hot condensate from the processes, then all the CO₂ and most of the O₂ will have been removed. The remaining amount of O₂ can be removed at a relatively low cost by dosing oxygen scavengers in the water. However, if the temperature of the condensate tank is below 80 °C in plants with insufficient returned condensate, large amounts of oxygen-scavenging chemicals will have to be dosed.

As a result, the amount of blowdown will have to be increased to keep the concentration of chemicals within the allowable limits. Due to the aforementioned reasons, deaerators can be used as a cheaper alternative. Compact deaerators, i.e., modern deaerators are designed by integrating a dome with trayed sections on top of the condensate tank, similar to the domed deaeration section above the storage vessel in conventional deaerators.

The compact deaerator is designed in such a way that the water in the condensate tank is uniformly heated at 85–90 °C at atmospheric pressure. With that design, the deaerator need not be elevated. The remaining O₂ can be removed using oxygen-scavenging chemicals. Ideally, the compact deaerator is used for processes in hospitals, hotels, or plants with non-elevated boiler rooms.